March 9th

COMMUNITIES

Community ecology studies how groups of species live together.

I. Terms

A. Niches can be defined from two different perspectives:

From the community point of view, a niche represents the role of a species in that community. As we will see, depending upon the nature of community structure, such 'roles' may not exist.

From the species point of view, a niche represents the range of environments that species lives in. This definition has become the more commonly used one by modern ecologists.

Using this last definition, two different types of niches may be defined:

(1) a Fundamental Niche is the range of environments a species could live in with no competitors, dispersal limitation, etc.

(2) a Realized Niche is the actual range of environments a species lives in when these other limiting factors are present.

Realized niches are always smaller than fundamental niches. In other words, because of competitors and other limiting factors, species live over a smaller range of environments than they could.

B. Guilds represent groups of species with similar niches in a community.

For instance, plants have been broken in to five guilds based upon the way they over-winter:

Trees and shrubs (buds held over 25cm above the ground); sub-shrubs (buds held <25 cm above the ground); rosette plants (bud held at ground level); bulb plants (bud held below ground level); and annuals (plant over-winters as a seed).

Ducks can also be broken down into guilds based on their food preferences. Dabblers are ducks which only shallowly dive for their food (primarily plants). Divers are ducks which deeply dive for fish, snails, and clams

C. Keystone Species are species whose role is so important that their presence is required for the health of the entire community. Examples:

Elephants graze woody vegetation, and keep the African Savanna open and grass-covered. Without elephants, the savanna would grow in with trees, and the many grass-eating herbivores (wildebeest, impala, etc.) would be lost.

Starfish eat mussels in tide pools in on the Pacific Coast. Without starfish, the mussels would take up the entire habitat. If this happened, many other species, such as sea anemones, would be lost.

II. Community Structure.

The way in communities of species live together have been viewed in two different very different ways.

A. Organismic View. This theory was advocated from the early 1900's to early 1960's. According to this theory, communities represented groups of highly co-evolved species. Each species has a unique and vital role in community function, and act with each other like the different organs do in our bodies. Because of this high level of connection between different species, communities are thought of a 'superorganisms'.

B. Individualistic View. This theory has been advocated from the mid-1950's to present. According to it, communities simply represent groups of species which have similar environmental requirements and because of that happen to live together. The species are not strongly linked, and do not represent 'organs' in a larger 'community superorganism'.

Work conducted in the 1950's in Wisconsin and the Great Smoky Mountains helped convince ecologists that the individualistic approach was more accurate.

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